Applying a film to a flat surface and spinning it on to a relatively uniform thickness is a technique that has been used in many environments, but is particularly common in the fabrication of integrated circuits since a uniform film can be applied to all parts of the circular wafer by placing a drop near the center and rapidly spinning the wafer. For many years, photoresist has been used as a spin on medium, and most recently actual glass, silicon dioxide (SiO.sub.2) which is used as a dielectric layer, as a passivation layer, and for planarization, has been employed for specific processes. While most spin on processes are similar in nature, each material has unique characteristics that affect the final properties of the film that results on the surface of the wafer.
Spin on glass (SOG) is a solvated form of silicon dissolved in an alcohol solvent. Like its name implies, SOG is spun onto a semiconductor wafer, similar to the manner in which photoresist is spun on, namely by putting a small quantity in the center of the wafer and spinning the wafer at a high rotational velocity to spread the substance over the surface of the wafer in a uniform film by centrifugal force. The spin operation spreads the SOG and establishes the bulk film characteristics while driving off solvents and binders through evaporation. Because of the evaporation of solvent, glass precipitates and deposits onto the wafer during the spin operation. The wafers are subsequently heat treated (annealed) to drive off the remaining solvents resulting in a film of glass.
Cracking or shattering occurs in spin on glass if the bulk thickness of the SOG film exceeds a certain characteristic threshold value. This condition is unfavorable because it results in defects in the finished wafer. For the bulk of the film, however, the thickness is easily controlled throughout the spin on operation and subsequent heat treatment. This is not the case, however, for the edge 10 of the SOG film 12 that occurs just above the round edge 14 of the wafer 16 in the region of edge shattering or cracking 18 as seen in FIG. 1. It will be appreciated that the thickness of the films and the wafers illustrated in the Figures are greatly exaggerated to show detail.
Uncontrolled cracking or shattering of the film occurs in this region 18 thus turning each wafer 16 into a source of particles during subsequent processing. It is well known in the integrated circuit industry that particles of any kind, especially of the scale that are generated by the shattering of the curved or thick edges of a spun on film, are extremely undesirable in the wafer processing environment and can destroy valuable integrated circuit product. Thus, removing the film on the very edge of the wafer is a requirement, regardless of the film type, for the particle-generating phenomenon can occur with any spun on material. While photoresist does not ordinarily shatter, nevertheless it tends to slough off at the edge and interfere with the circuit topography on the interior of the wafer.
Conventional spin on methods do not control the problem of edge shatter. Thus, as shown in FIG. 2, attempts have been made to remove the edge bead 10 that occurs near the rounded edge 14 by applying the method of back side rinsing or edge bead removal (EBR) to the spin on material operation. The edge bead 10 is the edge of the film 12 near the round edge 14 of the wafer 16. Edge bead removal refers to a process that is common in photoresist operations where the resist application is followed by spraying a solvent 20 on the back side 22 of the wafer 16 via symbolically represented nozzle 24. As the wafer 16 spins, the solvent 20 creeps over the outer edge 14 of the wafer 16 to remove the edge bead 10 of resist 12. Another means of EBR involves directing a jet of solvent at the wafer periphery while the wafer is spinning, as disclosed in U.S. Pat. No. 4,510,176. Attempts at edge bead removal of SOG, in contrast with photoresist, have been abandoned in the past because cracking was not eliminated. As shown in FIG. 3, past attempts served only to "push" the edge bead 26 back and pile the material up and simply relocate the site of the cracking 18. While a piled up edge bead can be detrimental if it is composed of photoresist, since it may slough off and interfere with further processing, as noted, known EBR techniques are somewhat successful with photoresist. However, the use of such methods on other materials such as SOG have failed and merely relocated the site of the particle generation.
Attempts to date have taken a single step approach using some type of alcohol to serve as the EBR solvent. In this case EBR is performed in a single step with little attention to the chemistry of the process or the fluid dynamics. This approach has resulted in processes that do not achieve the goal of reducing or eliminating shattering at the edge of the film. The above approach has been successful for photoresist operations where cracking and final defectivity of the film is not the critical issue. While photoresist is removed after each step, the spin on glass remains on the wafer permanently resulting in visual defects. The edge cracking of photoresist can cause blocked implants as well as deformation of etched patterns.
One interesting EBR technique is seen in U.S. Pat. No. 4,510,176 which describes a method for removing the edge bead region from a coated semiconductor wafer by directing a jet of solvent at the wafer periphery while the wafer is spinning. The flow patterns of debris resulting from this removal are controlled to prevent contamination of the chip sites on the wafer.